In recent years, sealed secondary batteries each in the form of a flat rectangular box have become widely used in portable electronic apparatuses and the like. An external container of one such secondary battery normally comprises an open-topped metallic container body and metallic sealing plate. The container body is in the form of a flat rectangular box. The sealing plate is welded to the container body, thereby closing its opening. In a manufacturing method for a sealed secondary battery, battery elements are first arranged in the container body before the sealing plate is welded to the container body. Then, the sealing plate is welded to the container body to form the sealed secondary battery. Subsequently, an electrolyte is injected into the container body through an injection hole formed in the sealing plate. When the injection of the electrolyte is finished, the injection hole is sealed with a sealing body in a reduced-pressure ambience.